The objectives of this study will be: 1. To further study thymic population shifts and the appearance of lymphoma cells in MuLV-M-carrier mice. 2. To isolate and clone additional lymphoma from these mice and to characterize them for their autoaggressive properties, their TdT and 20 a-SDh enzyme markers. 3. To further study the mechanism of autoaggression by MuLV-M-carrier thymocytes, including in vitro induction of aggressor cells. 4. To evaluate the need for H-2 antigen and ecotropic MuLV cell surface receptors for target cells to be susceptible to kill by MuLV-M carrier thymocytes; to evaluate the extent of MuLV expression required for target cell sparing. 5. To study thymocyte-mediated autoaggression and lymphomagenesis associated with the expression of endogenous MuLV in spontaneously high leukemic strain mice; to determine if possible the extent of genetic control over these expressions in crosses between high and low leukemia strain mice or in congenic high/low leukemia strain HRS/J mice.